(1) Field of the Invention
The present invention relates to a solid oxide fuel cell(SOFC) system, especially, to a large-sized SOFC system.
(2) Description of the Related Art
Fuel cells convert chemical energy of supplied gas into electric energy, therefore, offer great potential in efficient generation of electricity. Particularly, solid oxide fuel cells have attracted considerable attention as the fuel cells of the third generation following phosphoric acid fuel cells (PAFC) and molten carbonate fuel cells (MCFC) of the first and second generations, respectively.
The solid oxide fuel cells have an advantage in enhancing efficiency of generation of electricity, including waste heat management, with their operation at high temperature, approximately 1000.degree. C. Such an advantage is the basis for the study of enlargement of the solid oxide fuel cells in order to generate the mass current.
An electrolyte used in the solid oxide fuel cells is stabilized zirconium oxide (YSZ), that is, a product with defective fluorite structure obtained by doping yttrium oxide (Y.sub.2 O.sub.3) of approximately 8 mol% into zirconium oxide (ZrO.sub.2).
However, stabilized zirconium oxide retains low ion conductivity which is one of the factors that affects cell characteristics, therefore, it should be made into thin films in order to enhance the conductivity. On the other hand, inferior physical strength of the stabilized zirconium oxide films makes it quite difficult to produce enlarged solid oxide fuel cells. Accordingly, the study of electrolyte in its enforcement by mixing aluminum oxide (Al.sub.2 O.sub.3) therewith and its enlargement by having zirconium oxide deposit on an anode used as a supporter by vapor deposition (a substrate structure type) has been conducted, however, possible solutions for the enlargement of the solid oxide fuel cells are still in the quest. Moreover, even if a cell unit comprising one enlarged electrolyte is successfully produced, the accordingly extended distance between a gas inlet and a gas outlet contributes to an uneven temperature in a stack, and therefore, characteristics deterioration thereof.